A data transmission system is typically used to supply data between a transmitting station and a receiving station for the transmission of data to and between remote locations. As an example, an automatic meter reading system may use a data receiving station for supplying control signals to a plurality of remote meter units. The automatic remote meter units may operate to supply data pulses to the data receiving station indicative of use of a utility. A communication link may be employed which uses a pair of wires for supplying control signals to and for receiving data from the meter units.
Data transmission systems used for automatic transmission of meter data to a remote location typically utilize telephone or other data transmission media for the automatic reporting of meter or status data. In many such systems, the receiving station, upon receipt and accumulation of meter pulses, thereafter supplies an interrogation signal via the transmission medium to a reporting station to initiate the sending of a report. Such systems may involve ringing of the customer's telephone or the installation of speoial ring-suppress equipment at the customer's facility. Alternatively, special equipment at the telephone exchange may be utilized. In other types of systems, a reporting system initiates the making of a report.
Security for such transmission systems is critical to the integrity of the operation of the system and to prevent malfunctions of the system from various causes. For example, in systems for monitoring water, gas, or electric consumption at a residential or commercial location, a possibility of tampering with the metering equipment exists which prevents transmission of accurate data to a utility. Unauthorized individuals may attempt to alter data supplied by the automatic meter unit to reduce the consumption detected by the receiving station. Alternatively, attempts may be made to by-pass the meter unit altogether to eliminate any detection of consumption by the receiving station. In addition, normal operation of the system may be interrupted due to natural or unintentional causes.